1. Field of the Invention
The present invention relates to a robot system having a robot used for picking out and conveying a workpiece.
2. Description of the Related Art
A known system includes a robot used for picking out workpieces placed at unspecified positions, such as randomly piled workpieces in a container, by using a tool, and conveying the picked workpiece to a predetermined destination. In such a robot system, when the position and posture of the workpiece to be picked out are not accurately recognized, the position and posture of the workpiece is unknown. Therefore, a positional relationship between the workpiece and the tool cannot be specified at the time of holding the workpiece by the tool. More specifically, in the case where the workpiece is held in an unspecified manner, or the randomly-piled workpiece is held on a portion which is easy to hold, a portion of the workpiece which is held by the tool may vary every time. The posture of the workpiece relative to the tool when the workpiece is held by the tool may also vary every time. In these cases, as well as in the case where a position of the center of gravity of the workpiece cannot be determined even when the position and posture of the workpiece is known, it cannot be ensured that a positional relation between the tool and the center-of-gravity position of the workpiece becomes a desired one.
As a result, the workpiece may be held at a position distant from the center of gravity of the workpiece, or in an unstable posture. In this case, since the workpiece is not held stably, the position at which the workpiece is held by the tool may move relative to the tool, resulting in a change in the holding state of the workpiece, or the workpiece may fall during the conveyance, damaging peripheral devices. These may require the system to be temporarily stopped, and even pose a risk to workers in the surrounding area.
JP-A-2011-183537 discloses related art in which a holding state of the workpiece is checked, and if necessary, the workpiece is held again in a different posture. JP-A-5-241626, JP-A-2004-249391 and JP-A-2011-201007 disclose related art in which a posture of the workpiece relative to the tool is corrected by using a visual sensor or the like. JP-B-3300682 discloses related art in which operation of the robot is determined in accordance with a teaching model appropriate for picking out the workpiece by associating the image data of the workpiece obtained from a plurality of directions with the teaching model, and associating information on a direction between the robot and the workpiece relative to each other with the teaching model. JP-B-3925020 discloses related art in which when the robot fails to pick out a workpiece, the robot is operated to move the workpiece closer to the center of the container. JP-A-2000-263481 discloses related art in which when an abnormality is detected during the picking process of the workpiece, the picking process is suspended to retract the robot. JP-A-5-116081, JP-A-2012-40634 and JP-A-7-205075 disclose related art in which the weight and the center-of-gravity position of the workpiece, and the direction of the gravity force are calculated, in order to compensate an influence of the gravity force on a force acting between the tool and the workpiece.
According to the known related art described above, an additional step is required to hold the workpiece again when the holding state of the workpiece is unstable. This results in increased cycle time. Accordingly, there is a need for a robot system which allows the workpiece to be picked out stably, without need to hold the workpiece again once it is picked out.